Technical Field
The present invention relates to a polymer film, a method for producing a polymer film, a back sheet for a solar cell, and a solar cell module.
Background Art
Polyesters have been used for various applications such as electric insulation applications or optical applications. As electric insulation applications, solar cell applications in particular, such as a back face protective sheet (a “back sheet”) for a solar cell, have recently attracted much attention.
Further, polyesters generally have large numbers of carboxyl groups or hydroxyl groups on the surfaces thereof, which may result in frequent occurrence of hydrolysis reactions in an environment where moisture is present and in the degradation of polyesters over time. For example, solar cell modules are generally used in environments that are constantly exposed to wind and rain, such as outdoor environments; that is, solar cell modules are exposed to environments where hydrolysis reactions readily proceed. Therefore, in cases in which polyesters are used for solar cell applications, an important task is to suppress the hydrolyzability of polyesters.
As a technique for enhancing hydrolysis resistance and improving resistance to climatic conditions and heat resistance, a polyester film for a solar cell, which is obtained by adding an ultraviolet absorbent to a polyester obtained by polymerization using polycondensation catalysts including an aluminum-containing compound and a phosphorus-containing compound, and in which a prescribed carboxyl terminal concentration and IV are regulated, has been disclosed, for example (see, for example, Japanese Patent Application Laid-Open (JP-A) No. 2011-21180). In addition, a polyester resin sheet for a solar cell, in which a layer of a polyester resin having a prescribed number average molecular weight and a TiO2 layer are provided (see, for example, International Publication No. (WO) 2007/105306 pamphlet), a polyester film having a prescribed X-ray diffraction intensity ratio (see, for example, JP-A No. 2007-268710), a polyester film for a solar cell back face protective film, the polyester film being formed from a polyester containing an ethylene terephthalate unit and a polyester containing a butylene naphthalate unit, and having a prescribed intrinsic viscosity and plane orientation coefficient (see, for example, JP-A No. 2010-3900), and a hydrolysis resistant polyester film which includes a hindered phenol structural unit and has a prescribed acid value and intrinsic viscosity (see, for example, Japanese Patent No. 4678073), have been disclosed.
In all of these techniques, the breaking elongation after heating has attracted attention, and there have attempts to enhance the light resistance or heat resistance by improving this property.
Further, in the Patent Document (JP-A No. 2011-91303), the use of a PET for a back sheet, the PET having a relative temperature index of 100° C. and improved hydrolysis resistance, is described; however, the heat resistance thereof is resistance to temperatures not higher than 105° C. and the resistance to climatic conditions is insufficient, such that the improvements are essentially insufficient.
Moreover, from the viewpoint of hydrolysis resistance, the molecular weight of polyester is liable to be lowered significantly due to a hydrolysis reaction in a high humidity environment (“wet thermo conditions”), and since the lowering of molecular weight directly causes embrittlement, it is important to increase the breaking elongation of the polyester.